This invention relates to a glass frit composition, and particularly to a glass frit which is suitable for use in sealing window glass.
Display devices have been proposed which include a plurality of spaced parallel glass plates which are disposed between a back surface and a front viewing surface. The back surface includes a cathode area and the front surface includes a phosphor viewing screen. The glass plates are maintained in fixed position with an edge of each of the glass plates bonded to a common surface. The common surface may be a glass surface, e.g., either the front or back surface. It would be desirable to employ relatively inexpensive window glass both as the material for the glass plates and as the common surface. Window glass is generally characterized in the art as having an expansion coefficient of about 90 .times. 10.sup.-7 /.degree. C., hereinafter more simply referred to as 90. Soda-lime silicate glasses are among the typical window glasses.
In order to make the bonds or seals between the glass plates and the common surface, a sealing material is required. Many glass frit compositions exist which are useful in sealing or bonding various kinds of glasses. Among the most desirable are devitrifiable glass frits, such as those described in U.S. Pat. No. 2,889,952 entitled, "Composite Article and Method," issued June 9, 1959, to S. A. Claypoole. Devitrified glass is generally defined to mean that at least part of the glass is converted, or devitrified, to a crystalline phase and the remainder is retained in the vitreous phase. In this phase, the thermal and other physical characteristics of the devitrified glass, such as viscosity, density and expansion coefficient, may differ from those of the parent glass. In the case of a devitrified glass containing a high volume fraction of crystalline phase e.g., at least about 60 volume percent, the physical characteristics are substantially determined by the crystalline phase. As a result of the devitrified phase, a bond employing such a frit with a high crystalline content may be reheated to a temperature in excess of that at which the bond was formed, without causing softening or deterioration thereto.
However, the previously described display device presents three requirements particular to window glass which are not met by any known glass frit. First, the frit must be a low firing material, e.g., less than about 550.degree. C., while having physical stability at temperatures equal to or greater than its firing temperature. Second, it is desirable that the frit be fireable in an atmosphere which is reducing, inert, or oxidizing. It is particularly desirable that the frit be fireable in a reducing or inert atmosphere without becoming chemically reduced. Otherwise, the frit may become electrically conductive and develop conducting paths which can interfere with the electrical operation of the display device. This requirement eliminates all glasses which include lead oxide as a major constituent. Third, it is desirable that the vitreous (non-crystalline) and devitrified (crystalline) forms of the glass have expansion coefficients which are compatible with that of window glass. The third requirement reduces the likelihood of stress which could easily develop during the sealing process as the glass is converted from a vitreous body to a devitrified body.